In a typical fuel cell configuration, an electrolyte is sandwiched between electrodes (specifically, an anode and a cathode) such that positive ions generated at the anode flow through the electrolyte and react with negative ions generated at the cathode, while current generated by the flow of free electrons produced at the anode during the oxidation of the anode reactant and consumed at the cathode during the reduction of the cathode reactant can be used to power one or more external devices. Collectively, the anode and cathode, which are typically made of a porous carbon-based substrate material, are called electrodes. Such porous construction and relatively low-cost material allows wet gas permeation, provides a high surface area reaction surface against the electrolyte, is non-corrosive and is conductive to the free electrons that flow between the electrodes. Together, the electrolyte and the electrodes make up what is commonly referred to as the membrane electrode assembly (MEA). One form of fuel cell, called the proton exchange membrane or polymer electrolyte membrane (PEM) fuel cell, is particularly well-suited for vehicular and related mobile applications, as the solid polymer electrolyte (which is typically made from a plastic-like film of a perfluorosulfonic acid, such as Nafion®) is of a robust, relatively simple construction that can operate at relatively low temperatures.
An exploded, sectional view of a portion of a PEM fuel cell stack is shown in FIG. 1. It includes the MEA made up of the proton exchange membrane 12 sandwiched between an anode catalyst layer 14 and a cathode catalyst layer 16. In addition, an anode diffusion layer 18 and a cathode diffusion layer 20 are placed in an adjacently facing relationship to the MEA such that the anode catalyst layer 14 and a cathode catalyst layer 16 provide substantial surface contact with both the proton exchange membrane 12 and the respective diffusion layers 18, 20. As stated above, the diffusion layers 18, 20 form a high surface area catalytically active substrate. For example, these can be made of refractory metal oxides, activated carbon, graphite or the like. Bipolar plates 22 engage the anode diffusion layer 18 and the cathode diffusion layer 20, and include lands 25 that separate adjacent sections of reactant gas flow channels 24, 26. In a general (although not necessary) form, the cathode diffusion layer 20 may be thicker than the anode diffusion layer 18 to make it more difficult for water vapor to travel quickly through the thickness of the layer. This in turn produces a water vapor concentration gradient within the cathode diffusion layer 20 to maintain the proton exchange membrane 12 in a sufficiently hydrated state.
To facilitate the ionization of the respective anode and cathode fuels in PEM fuel cells, a noble metal catalyst is deposited on the surface of the electrodes. Platinum (Pt) is the most common example of such a catalyst, and due to its high electrocatalytic activity, stability and electrical conductivity, it provides a ready chemical reaction site without being consumed in the process. Unfortunately, platinum is very expensive, so the amount used is a significant portion of overall fuel cell cost. Consequently, one of the major challenges facing the commercial application of fuel cells is the reduction in the amount of platinum used.
The sluggish kinetics of an oxygen reduction reaction (ORR) at the cathode, particularly at low temperatures, require that a large amount of platinum be used. Moreover, in order to maximize the catalytic activity, the platinum is generally fabricated as very fine particles. The particle size for catalysts deemed most appropriate for vehicular use is typically between about 2 and 5 nanometers (nm) in diameter. The small particle size allows these catalysts to achieve a high specific surface area (i.e., the active platinum area per mass of platinum). However, as the particles become very small (for example, smaller than about 2-3 nm), both the ORR activity and the durability deteriorate in a phenomenon known as the platinum particle size effect. This is often ascribed to the increase in low coordination number surface atoms on the kinks and edges of the particles. The small particle size makes the particles susceptible to area loss during use via platinum dissolution and redeposition, (Ostwald ripening).
A continuous platinum film provides higher stability against dissolution. Moreover, a platinum film gives higher specific ORR activity per unit surface area of platinum compared to platinum as nanoparticles. For example, low-platinum catalysts take advantage of the observation that large, smooth surfaces of bulk platinum give 5 to 10 times the ORR activity of platinum nanoparticles when normalized per surface platinum atom. The problem with large, smooth platinum surfaces is that the vast majority of the atoms are buried beneath the surface. The growth of smooth, very thin platinum layers (e.g., less than 10 monolayers, or 2.2 nm) on a smooth, inexpensive substrate could produce catalysts that provide high activity per surface platinum atom as seen for large bulk platinum electrodes and a sufficiently high proportion of the total platinum atoms residing on the surface to give a high activity per mass of platinum used.
However, growing continuous metal films at such a small thickness has posed great challenges. When platinum is deposited on most substrates, it usually forms a three-dimensional cluster in order to maintain its extraordinary high surface energy (about 2.5 J/m2).
Atomic layer deposition (ALD) is a technique for the deposition of thin metal films based on sequential, self-limiting surface reactions. The ideal characteristics of ALD are atomic layer control of the thin film thickness and conformality on the underlying substrate. Many ALD systems display these ideal characteristics. One important example is Al2O3 ALD using trimethylaluminum and water. Al2O3 ALD can deposit extremely conformal films on high aspect substrates, as well as on nanoparticles.
Metal ALD has developed rapidly, and many metals can now be deposited using ALD, including important catalytic metals such as platinum and ruthenium (Ru). One difficulty that has been observed for some metal ALD systems is the inability to nucleate easily on some substrates, such as oxide substrates. For example, platinum ALD using reductive elimination chemistry with MeCpPtMe3+O2 as the reactants has nucleation difficulties on SiO2 and Al2O3 substrates. No platinum ALD is observed for hundreds of ALD cycles, and when it can finally be observed, the deposition is in the form of nanoclusters. These nanoclusters may eventually grow together to form a continuous film after more ALD cycles.
The thickness at which the nanoclusters grow together to form a continuous film is much larger than 1 nm and probably larger than 5 nm, making them too thick for many ALD film applications. For example, the efficient use of expensive catalytic materials such as platinum is required in ultrathin thicknesses to reduce the cost. Thin layers of metals in various structures are also required for magnetic multilayers. For proper operation, the thicknesses of these metal layers need to be less than the thickness at which the metal layers become continuous.
The difficulty for the nucleation of metals on many substrates such as oxide surfaces is the large difference between the surface energy of the metal and the surface energy of the substrate. Metals such as platinum have large surface energies on the order of about 2.5 J/m2. Oxide supports such as Al2O3 have much lower surface energies, for example, about 1.8 J/m2. The result is that the metals will sinter into nanoclusters if they have sufficient surface mobility to reduce their surface energy. The surface energy is minimized because a three-dimensional metal nanocluster has a lower surface area than a two-dimensional metallic film containing the same number of atoms as the nanocluster that covers the underlying substrate.
One solution to the difficulty of metal nucleation is to deposit the metal onto a substrate that has a much higher surface energy than the metal itself. In this case, the metal will want to form a two-dimensional continuous film on the underlying substrate because such a deposition geometry will reduce the surface energy. The extremely high surface energy of tungsten (˜3.5 J/m2) is such that platinum deposited on tungsten would be expected to form two-dimensional continuous films on the underlying tungsten substrate.
Tungsten-based ALD can nucleate rapidly on Al2O3 ALD substrates. Such a structure is performed using WF6 and Si2H6 as the reactants, where under optimized conditions, a continuous and ultrathin tungsten ALD film can be obtained on Al2O3 ALD at a thickness of only 2.5 nm. On the basis of surface energies, this tungsten ALD film should sinter and form nanoclusters on the Al2O3 ALD surface. However, the tungsten ALD surface chemistry is very exothermic and can be performed at low temperatures. At these low temperatures, the tungsten atoms have very low surface mobility and do not sinter to form nanoclusters.